1. Field of the Invention
The present invention relates to a plasma processing apparatus which uses a plasma for forming a thin film on a surface of a workpiece or for etching a surface of a workpiece to fabricate a semiconductor device.
2. Description of Prior Art
FIG. 6 shows a schematic sectional view of an example of a conventional dry etching apparatus disclosed in JP-A No. 7-78805. In FIG. 6, a lower RF (radio-frequency) electrode plate 11 is disposed in a lower portion of the interior of a vacuum vessel 10. The lower RF electrode plate 11 is disposed not to contact directly with the vacuum vessel 10, but is electrically isolated from the vacuum vessel 10 by an insulating member 21. An upper RF electrode plate 12 is connected electrically to the vacuum vessel 10 and is kept at a ground potential. A semiconductor wafer 13, i.e., a workpiece, is mounted on the lower RF electrode plate 11. A reactive gas inlet 14 is formed in the upper wall of the vacuum vessel 10. A reactive gas, i.e., a process gas, is supplied through the reactive gas inlet 14 into the vacuum vessel 10. A reactive gas outlet 15 is formed on the bottom wall of the vacuum vessel 10. The reactive gas is discharged from the vacuum vessel 10 through the reactive gas outlet 15. The lower RF electrode plate 11 is internally provided with a refrigerant pipe 16 for cooling the lower RF electrode plate 11 so that the lower RF electrode plate 11 is held at a fixed temperature. The refrigerant pipe 16 has one end serving as a refrigerant inlet 17 and the other end serving as a refrigerant outlet 18. The lower RF electrode plate 11 is connected to a RF power supply 20 by a coupling capacitor 19. In the vacuum vessel 10, a plasma 7 is generated between the lower and upper electrodes 11 and 12.
In operation, a reactive gas is supplied through the reactive gas inlet 14 into the vacuum vessel 10, while the vacuum vessel 10 is evacuated through the reactive gas outlet 15 so that the interior of the vacuum vessel 10 is kept at a predetermined pressure. Then, an RF voltage is applied to the lower RF electrode plate 11 by the RF power supply 20. Consequently, the discharge occurs between the parallel RF electrode plates 11 and 12 to produce a plasma in a space between the parallel RF electrode plates 11 and 12. The plasma 7 thus produced is used for dry-etching the semiconductor wafer 13.
In the conventional plasma processing apparatus thus constructed, the plasma 7 is produced in a space between the lower RF electrode plate 11 and the upper RF electrode plate 12. Therefore, the conventional plasma processing apparatus has an advantage of producing a uniform plasma in a large area by a simple arrangement. However, the conventional plasma processing apparatus has the following disadvantages.
In order to form a minute pattern accurately by dry-etching on the semiconductor wafer 13, anisotropic etching must be achieved by using a reactive gas at a low pressure to increase particles which fall on the semiconductor wafer 13 perpendicularly to the surface of the semiconductor wafer 13. For example, according to Kazuo Akashi, "Optics Plasma Processing", Nikkan Kogyo Shimbun-sha, p. 234, an appropriate pressure of the reactive gas for anisotropic plasma etching is about 1.33.times.10.sup.-3 Torr).
However, in the conventional plasma processing apparatus, the pressure range of the reactive gas is limited. The discharge occurs between the parallel RF electrode plates 11 and 12 when RF power is supplied to the parallel RF electrode plates 11 and 12. A parallel-plate plasma etching apparatus needs a relatively high gas pressure to sustain discharge and, for example, according to Kazuo Akashi, "Optics Plasma Processing", Nikkan Kogyo Shimbunsha, p. 234, a suitable gas pressure is in the range of 13.3 to 1.33 Pa (1.times.10.sup.-1 to 1.times.10.sup.-2 Torr).
Therefore, dry etching cannot be carried out in an environment of a high vacuum in which the gas pressure is relatively low.